In the recently issued patent to Jacobs 3,785,941 (assigned to Aluminum Company of America) there is described a refractory material for the manufacture of electrolytic cells in which aluminum is produced by the electrolysis of aluminum chloride in a molten salt mixture of sodium chloride, lithium chloride and aluminum chloride. A preferred refractory for such cells is described as being silicon oxynitride (Si.sub.2 ON.sub.2) since it has the ability to resist the highly corrosive atmosphere resulting from the presence, in the electrolysis cell, of the molten salts, molten aluminum and chlorine gas. Such silicon oxynitride can be made in accordance with Washburn U.S. Pat. Nos. 3,356,513, 3,639,101 and 3,679,444 assigned to Norton Company. While the above patents provide a product which is extremely resistant to the environment existing in the electrolysis cell, the product has a thermal conductivity which is relatively high, being on the order of 35 to 40 BTU inch/hr./ft..sup.2 /.degree.F. at a density of about 75% of theoretical thereby undesirably increasing the heat loss from the electrolysis cell during its operation.
Accordingly, it is a principal object of the present invention to provide a refractory silicon oxynitride shape which is resistant to the kind of corrosive atmosphere present in aluminum chloride electrolysis cell but which has a greatly decreased thermal conductivity.